Non-destructive evaluation (NDE) is well known for inspecting parts or materials for defects such as porosity or inclusions like gas bubbles or foreign material. For example, x-rays are used for NDE of manufactured parts for systems (such as piping systems), structures, and vehicles (such as ships, land vehicles, and air vehicles, e.g., aircraft and spacecraft). As a further and more specific example, because aluminum castings are used extensively in aircraft manufacturing, inspection costs for NDE of aluminum castings represent a significant percentage of total inspection costs for an aircraft.
Such use of x-rays is sometimes referred to as radiography. Traditionally, radiography techniques entailed placing a part to be examined for defects between an x-ray source and a silver-halide film. However, traditional silver-halide film techniques are costly and time-consuming.
A reduction in costs and time associated with traditional silver-halide film techniques has been achieved with digital radiography techniques. Digital radiography encompasses a wide range of technologies, including flat panel technologies, computed radiography, and a variety of scintillator and digital camera-based technologies. In switching from traditional film radiography to digital radiography, costs associated with purchasing film, processing film, and chemical waste disposal can be eliminated. In addition, significant savings can be realized through cycle time reduction and automation that digital radiography offers.
A major hurdle to implementation of digital radiography for a number of industrial uses is a lack of digital reference images. Many differences result in a radiographic image captured on film versus a radiographic image captured by any of the numerous digital modalities currently available. As a result, direct comparison of digital radiographs to current reference radiographs captured on film leads to different characterizations of severity levels between the various digital modalities and between digital and film radiography.
An attempt has been made within the aluminum casting industry to address these differences by converting existing film reference radiographs (ASTM E155 Reference Radiographs for Inspection of Aluminum and Magnesium Castings) to digital images. However, traditional film reference radiographs such as ASTM E155 do not translate directly into the domain of digital radiography. The existing reference radiographs have proved inadequate for two main reasons: (1) the difference in spatial resolution between radiographic film and the digital radiographic systems; and (2) the difference in dynamic range between film and many of the digital detectors.
Regarding the first shortcoming in use of existing film reference radiographs, the grain size of common radiographic film ranges in size from about 3 microns (μm) to about 10 μm, while pixel spacing for digital radiographic systems that are suitable for inspection of aluminum castings, without use of geometric magnification, range from about 50 μm to about 139 μm. These differences in resolution do not seem to affect the detection of a discontinuity. However, these differences in resolution do affect the grading of the severity level of a discontinuity. For example, FIG. 1 is a film radiograph 10 of plate three of elongated porosity ¼″ (ASTM E155) that was digitized with a pixel spacing of 140 μm; FIG. 2 is a film radiograph 20 of plate five of the same series digitized with a pixel spacing of 50 μm; and FIG. 3 is a film radiograph 30 that is the same as the film radiograph 10 (FIG. 1) only digitized at 50 μM. It can be seen through a comparison of the film radiographs 10, 20, and 30 that a difference in resolution of a detector (or in this case digitization pixel size) between 50 μm and 140 μm results in a shift of approximately two plates in the apparent severity level.
Regarding the second shortcoming in use of existing film reference radiographs, the difference in dynamic range between film and many of the digital detectors has shown that use of ASTM E155 reference radiographs is inadequate for the grading of the severity level of aluminum castings. The wide dynamic range of digital detectors, coupled with the limitation on a number of gray level intensities that humans can differentiate, makes it necessary to step through the data of a given image with a series of windows. This is currently done by adjusting the contrast (window width) and then changing the brightness (window level) in a series of steps to view the data.
The shortcoming with this approach arises when the contrast of a production radiograph taken with a digital detector is adjusted. When using a high contrast, the discontinuity looks worse (that is, a higher plate number). When using a low contrast, the discontinuity may not be visible at all. For example, this effect is shown in FIGS. 4-6. All of the images are from a single 16 bit dynamic range digital radiograph of the ASTM E155 hardware for ¼″ elongated porosity in aluminum. FIGS. 4 and 6 are digital radiographs 40 and 60, respectively, with the same contrast setting and FIG. 5 is a digital radiograph 50 at a slightly higher contrast setting. In comparing the digital radiograph 40 (FIG. 4) to the digital radiograph 50 (FIG. 5), there is a noticeable difference even though they are both of plate two and from the same radiograph. Instead, the digital radiograph 50 (FIG. 5) looks more like the digital radiograph 60 (FIG. 6). However, the digital radiograph 60 (FIG. 6) is of plate seven-a difference of five severity levels from plate 2 (see FIGS. 4 and 5).
Because it has not been possible to normalize contrast of the standard reference image relative to the part's image using known methods, direct comparisons between the standard reference image and the part's image have not been possible. Development of digital reference images would therefore be useful to capture the savings offered by digital radiography. However, there is an unmet need in the art for methodologies to use a set of digital reference images.